In thermophotovoltaic (hereinafter, "TPV") energy conversion, radiation from heated surfaces is intercepted by photovoltaic (hereinafter, "PV") cells. If the PV cells are chosen so that their energy gap for charge separation is matched to the energy of the intercepted radiation, electricity will be generated.
There have been several efforts to concentrate thermal radiation through the use of mirrors (specular reflectors) analogous to those used to concentrate sunlight to decrease the cell area necessary for PV conversion. U.S. Pat. No. 5,312,521 to Fraas and a reference document to U.S. Pat. No. 5,312,521 "The Design and Development Tests of Direct-Condensing Potassium Radiators" draw from this work. Also devices have been proposed to incorporate a reflector for reflecting a second portion of the incoming energy to the thermal radiator (U.S. Pat. No. 4,746,370 to Woolf). These devices utilize specular mirrors to reflect and redirect incident photons.
There are important differences between sunlight and the radiation from practical thermal sources, however. Direct sunlight is almost a parallel beam, that is, it is characterized by a small angle. Since the angle between light reflected from a specular mirror and the perpendicular to the mirror is the same as the angle between the incident light and the perpendicular to the mirror, direct sunlight reflected from a specular mirror will also subtend a small angle, and thus it may be possible to design a mirror so that it will redirect sunlight onto a small concentration area. The theoretical limit to how much radiation within a certain angle.sub..crclbar. may be concentrated in two dimensions is 1/(sin.sub..crclbar.)2.
Thermal radiation on the other hand tends to have a large angular spread, .crclbar./, in fact it is isotropic for black bodies. Thus, when radiation characterized by a wide angle spread is reflected by a specular surface, the reflected radiation will also be spread over a wide angle. In the limit of isotropic radiation incident, the theoretical limit of concentration is one. That is, concentration is not possible.
Another major difference between sunlight and thermal radiation has to do with power densities. Because of the large distance from the sun to the earth, the maximum intensity for sunlight on the surface of the earth is close to 1.3 kW/m 2, which is not only several orders of magnitude smaller that the irradiance from a black body hot enough to emit radiation at a spectrum approximating sunlight, but several orders of magnitude smaller than the irradiance from a gray body which is hot enough to emit appreciable radiation at the lowest photon energies which will generate charge separation in PV cells. Because no mirror is perfect, the absorber fraction will tend to heat the reflecting surface to temperatures which are too high for traditional reflectors, i.e. mirrors, lenses, and holograms. This is a consequence of Kirchoffs Law, which states that if a surface is a poor absorber at a certain wavelength, it will be a poor emitter at the same wavelength and angle.
From these considerations it appears that conventional mirrors are not attractive redirectors of thermal radiation for TPV devices--and neither are lenses or holograms. Gold may be the one exception to this statement, but gold is cost prohibitive for many TPV applications. These arguments can be augmented with the observation that in many TPV conversion devices the emitting reflecting and converting surfaces may be exposed to reducing and reactive atmospheres.